This invention relates to gear pumps and, more particularly, to gear pumps which are specially suited for use in a polymerization process or an extrusion molding line for all kinds of non-Newtonian fluid such as molten resins like polyethylene and polystyrene.
As shown in FIG. 3, a prior art gear pump for transporting non-Newtonian fluids has a pair of flat gears 6a and 6b having mutually intermeshing teeth and contained inside a casing 2. The casing 2 is comprised of a housing 3 having its open ends on both sides sealed with a front cover 5 and a rear cover 4. The gear shaft 7a which supports one of the gears (6a) is connected to a power source (not shown) and will be referred to as the drive gear shaft. The gear shaft 7b around which the other gear 6b rotates will be referred to as the driven gear shaft. These two gear shafts 7a and 7b are rotatably supported by bearing members 8a and 8b, respectively, which also serve as side plates for sealing the side surfaces of the gears 6a and 6b.
The drive gear shaft 7a penetrates both the front cover 5 and the rear cover 4, extending outward. Gland cases 12 are provided where the drive gear shaft 7a penetrates the front and rear covers 5 and 4 so as to prevent the fluid from leaking out of the casing 2. Numerals 11 in FIG. 3 indicate where the drive gear shaft 7a penetrates the front and rear covers 5 and 4. A suction port IN and a discharge port OUT are provided where the mutually intermeshing teeth of the gears 6a and 6b sequentially disengage from each other and engage with each other, respectively, as rotary power is applied to the drive gear shaft 7a such that a fluid is sucked in through the suction port IN and discharged out through the discharge port OUT in a well-known manner.
Liquid-pooling grooves 9a are provided on the inner peripheral surfaces of the bearing members 8a and 8b, reflux grooves 9b are provided on the inner surfaces of the covers 4 and 5, and reflux holes 9c are provided on the suction side of the casing 2 such that a portion of the fluid flowing out through the discharge port OUT will return to the oppositely situated suction port IN by passing through the liquid-pooling grooves 9a, reflux grooves 9b and reflux holes 9c, lubricating the bearing members 8a and 8b at the same time. Another portion of the fluid will flow through the side gaps S.sub.x between the side surfaces 6c of the gears 6a and 6b and the bearing members 8a and 8b in the direction normal to the page of the figure, returning from the discharge port OUT to the suction port IN, serving to lubricate the bearing members 8a and 8b and to slidingly seal the side surfaces of the gears 6a and 6b.
With a gear pump thus structured, however, it is difficult to accurately estimate the internal leakage and the power loss at the time of assembly, and errors are likely to occur in the discharge rate and required power at the time of actual operation. As a result, the user is likely to experience insufficiency in lubrication and insufficiency in power.
One of the reasons for this problem may be explained as follows. The magnitude of leakage per unit time q.sub.s at the side gaps S.sub.x is given by: EQU q.sub.s =(k.sub.1 +k.sub.2)PS.sup.3 /.mu. Formula (1)
where k.sub.1 and k.sub.2 are constants determined by the design of the pump, p is the pressure of the discharged fluid, .mu. is the viscosity of the fluid (that is, its kinetic viscosity related to its motion) and S is the width of the side gaps. The power loss w.sub.s of the gear pump due to the friction on the side surfaces 6c of the gears 6a and 6b is given by: EQU w.sub.s =(k.sub.3 +k.sub.4).mu.n.sup.2 /S Formula (2)
where k.sub.3 and k.sub.4 are constants and n is a coefficient determined by the design of the pump. On the other hand, the viscosity .mu. of a non-Newtonian fluid varies according to the speed of fluid motion (in the direction perpendicular to the page of FIG. 1) .gamma. of the fluid typically as shown in FIG. 5, but the speed of fluid motion .gamma. at a point at radial distance D from the shaft 7a or 7b is given by EQU .gamma.=.pi.DN/S Formula (3)
where N is the number of rotation per unit time of the shafts 7a and 7b. FIG. 5 indicates that .mu. increases as .gamma. decreases, and since the width S of the side gaps is constant radially (that is, independent of radial distance D) in the case of a prior art pump as shown in FIG. 3, this means that the kinetic viscosity .mu. of the fluid is lower at points at a larger radial distance D (or farther from the shaft 7a or 7b).
Accordingly, the results of calculations for leakage q.sub.s and power loss w.sub.s will vary significantly, depending on what value is used as the kinetic viscosity .mu.. One may attempt to simply take the average value between the maximum and the minimum, but the fluid does not flow smoothly if the speed of fluid motion .gamma. varies throughout the side gaps S.sub.x, and the actual viscosity of the fluid is likely to be very different from a theoretically calculated value. In other words, it is very difficult to design a pump which actually functions as theoretically predicted.